Method and apparatus for charging by corona discharge

ABSTRACT

In a corona discharge device for charging the surface of a body, for example a photosensitive element utilized in the art of electrophotography, the corona discharge current is controlled to vary at a predetermined rate at the commencement and termination of corona discharge so as to ensure uniform deposition of electric charge on the surface of the element.

United States Patent Watanabe et al.

METHOD AND APPARATUS FOR CHARGING BY CORONA DISCHARGE Inventors:Masanori Watanabe; Takaaki Konuma; Hirokazu Asano, all of Tokyo, JapanKabushiki Kaisha KIP, Tokyo, Japan Filed: Mar. 14, 1974 Appl. No.:451,008

Assignee:

Foreign Application Priority Data Mar. 30, 1973 Japan 48-36293 US. Cl.317/262 A; 250/325 Int. Cl. H01T 19/00 Field of Search 317/262 A;

IQ DRIVING CIRCUIT l2 CURRENT 5 REGULATOR j SOURCE Primary ExaminerJ. D.Miller Assistant ExaminerI-larry E. Moose, Jr. Attorney, Agent, orFirmBosworth, Sessions & McCoy [57] ABSTRACT In a corona dischargedevice for charging the surface of a body, for example a photosensitiveelement utilized in the art of electrophotography, the corona dischargecurrent is controlled to vary at a predetermined rate at thecommencement and termination of corona discharge so as to ensure uniformdeposition of electric charge on the surface of the element.

8 Claims, 9 Drawing Figures l4 HIGH VOLTAGE a l l SYNCHRONIZING CIRCUIT4 CORONA DISCHARGE DEVICE l3 TIME CONTROLLER U.S. Patent 001. 14, 1975CORONA DISCHARGE Sheet 1 of 2 3,912,989

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IQ DRIVING CIRCUIT I2 CURRENT l4 HIGH VOLTAGE I REGULATOR 1 SOURCE L 1/ll SYNCHRONIZING 4 gi mg CIRCUIT b RI DEWCE M QwQ/R l3 TIME CONTROLLERMETHOD AND APPARATUS FOR CHARGING BY CORONA DISCHARGE BACKGROUND OF THEINVENTION This invention relates to a method and apparatus fordepositing electric charge on a surface of relatively large area bymeans of corona discharge, and more particularly to a method andapparatus for uniformly depositing electric charge in a short time onthe surface of the photosensitive element of an electrophotographicapparatus on which an electrostatic latent image is to be formed.

In the art of electrophotography or the like wherein it is necessary tocharge the surface of a body to be charged, e.g. a photosensitiveelement having a relatively large area, it has been the practice toprovide a corona discharge device including a plurality of coronadischarge wires confronting the image forming surface of the element andto move the corona discharge device in parallel with and relative to theimage forming surface. The corona discharge device utilized in thismethod is usually constructed such that at least two corona dischargewires pass by respective points on the image forming surface during thecorona discharge interval so as to prevent non-uniform distribution ofthe deposited charge and to prevent the formation of the shadows of thedischarge wires, but where a large quantity of charge is to be depositedin a short time there is a defect in that non-uniform distribution ofthe deposited charge in the form of stripes parallel to the coronadischarge wires often results. We have found that this defect is causedfor the following reason. More particularly, it has been well known thatwhen a high voltage is impressed upon an ordinary image forming surface,a large charging current flows instantaneously through thephotosensitive element at the time of commencement of chargingirrespective of the position of the corona discharge device. For thisreason, portions of the image forming surface which were confronting thecorona discharge wires at the time of commencement of discharge receivemore charges than those remote therefrom, thus resulting in nonuniformcharge distribution. Although the operation at the time of terminationof the deposition of charge on the image forming surface is not yettheoretically established, we have noted that there is a substantialdifference between the quantities of the charges deposited on theportions immediately beneath the corona discharge wires and on theportions remote therefrom when the deposition of the charge isinterrupted while the corona discharge device is moving along the imageforming surface or is maintained stationary. We have noted that thesephenomena of nonuniform charge distribution occurring both at the timeof commencement and interruption of the corona discharge are enhancedwhen the voltage impressed upon the corona discharge device is increasedfor the purpose of increasing the quantity of the charge to bedeposited.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto provide a new and improved method and apparatus for uniformlydepositing electric charge on a surface of a relatively large area, suchas the image forming surface of a photosensitive element utilized inelectrophotography.

According to one aspect of this invention there is provided a method ofcharging the surface of a body to be charged by corona discharge whereinthe surface is charged by means of a corona discharge devicesubstantially covering the entire surface of the body and by relativelymoving the body and the corona discharge device, characterized in thatthe current flowing through the body is varied at a predetermined rateduring the period of commencement of the corona discharge until apredetermined current value is reached and that the current is graduallydecreased from the predtermined current value during the period oftermination of the corona discharge.

According to another aspect of this invention there is providedapparatus for charging a body to be charged by means of corona dischargeof the type comprising a corona discharge device substantially coveringthe surface of the body, a source of power for energizing the coronadischarge device, and means for moving the corona discharge devicerelative to the body in a direction substantially parallel to thesurface of the body, characterizedin that there is provided controlmeans for controlling the corona discharge current of the coronadischarge device, and that the control means includes means for varyingthe corona discharge current at a predetermined rate at the time ofcommencement of the corona discharge until a predetermined current valueis reached and means for gradually decreasing the corona dischargecurrent from the predetermined current value at the time of terminationof the corona discharge.

The control means may be an electrical control circuit or a mechanicaldevice.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of theinvention can be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a diagrammatic representation showing an arrangement of aconventional corona discharge device and a photosensitive element;

FIG. 2 is a graph showing the waveform of the current flowing throughthe grounded plate of the corona discharge device shown in FIG. 1;

FIG. 3 is a graph showing the waveform of the current flowing throughthe photosensitive element shown in FIG. 1;

FIG. 4 is a block diagram showing one example of the control deviceutilized to carry out the invention;

FIG. 5 is a graph similar to FIG. 2 and shows the current waveformobtained by carrying out the method of this invention;

FIGS. 6, 7, and 8 are graphs similar to FIG. 3 and show currentwaveforms of the current flowing through the photosensitive element whenthe method of this invention is applied; and

FIG. 9 is a diagrammatic perspective view showing a modified embodimentof this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS To have better understanding ofthe invention, prior art method and apparatus will first be describedwith reference to FIG. 1. The body to be charged by corona dischargeshown therein comprises a photosensitive element 30 utilized inelectrophotography and including an insulative layer 2, a photosensitivephotoconductive layer 3 and a grounded electrode layer 1 which arebonded together into an integral structure. According to certain typesof electrophotography, the insulative layer 2 is used as an imageforming surface and is deposited with electric charges by means of acorona discharge device 4 spaced apart a short distance from theinsulative layer 2 and moved in parallel with the surface thereof. Thecorona discharge device 4 has a size sufficient to cover the entiresurface of the insulative layer 2 for the purpose of depositing chargein an extremely short time and includes a grounded conductive substrate5, a plurality of equally spaced corona discharge wires 6 insulated fromthe substrate and a plurality of parallel conductive vanes 7 eachinterposed between adjacent discharge wires 6 and electrically connectedto the substrate 5. In some types of electrophotography, a light imageis projected upon the photoconductive layer 3 through the coronadischarge device 4 in which case the substrate 5 is made of a conductiveglass plate such as Nesa Glass (registered trade mark) or takes the formof a rectangular frame. In another type of electrophotography, a lightimage is projected through the electrode layer in which case theelectrode layer 1 must be transparent to light. The corona dischargewires 6 are connected to the negative or positive terminal of a DCsource 9 via a switch 8 depending upon the conductivity type of thephotosensitive element 30. The opposite terminal of the source 9 isgrounded as shown.

To deposit a charge of negative polarity, for example, on the insulativelayer, the switch 8 is closed to create corona discharge between coronadischarge wires 6 and vanes 7 and or substrate 5 while the coronadischarge device 4 is being moved at a constant speed in parallel withthe surface of the insulative layer 2 thereby depositing negative chargethereon. A predetermined interval later, switch 8 is opened and themovement of the corona discharge device 4 is stopped thus completing thedeposition of the charge. The current flowing through the groundedsubstrate 5 of the corona discharge device 4 during this interval isshown by the graph shown in FIG. 2 in which t represents an instant atwhich the corona discharge was commenced and t an instant at which thecorona discharge was terminated. As shown, the current is maintained ata constant value between instants t and The graph shown in FIG. 3 showsthe variation in the current flowing through the photosensitive elementduring the interval between instants t and As shown, a large chargingcurrent I flows through the photosensitive element 30 at the instant twhen the corona discharge is initiated and the current graduallyattenuates to a small value I, at the instant I at which the coronadischarge is terminated. Thereafter, the current rapidly decreases tozero. The reason that the current does not instantly decrease to zero atinstant is caused by the fact that it takes a certain time fordeionization.

These curves show the result of the following experiment. In thisexperiment, the photosensitive photoconductive layer was omitted and abody to be charged was comprised by a polyester resin film having athickness of 16 microns and an area of 300 X 300 mm and the electrodelayer consisted of an aluminum layer vapour deposited on the rearsurface of the polyester resin film. Each corona discharge wire had adiameter of 0.06 mm, the spacing between the corona discharge wires andthe polyester resin film was selected to be 7 mm. The corona dischargedevice was moved at a speed of mm/sec, and operated for 0.5 sec. underan applied voltage of 4,60OV and a total current of 60 mA. When thecharge deposited on the surface ur the insulative layer was developed orvisualized by sprinkling a toner which is used to develop electrostaticimages in the art of electrophotography over thhe surface of thepolyester resin film, clearly defined stripes were noted on the portionsof the polyester resin film which were positioned immediately beneaththe corona discharge wires at the times of commencement t andtermination of the corona discharge t showing nonuniform distribution ofthe deposited charge.

From the result of this experiment, we have noted that such a nonuniformdistribution of the charge can be prevented by controlling the currentflowing through the member to be charged at the times of initiation andtermination of the corona discharge.

FIG. 4 shows a connection diagram of one example of a current controldevice that can be used in combination with the corona discharge deviceshown in FIG. 1. This control device comprises a driving circuit 10which operates to move the corona discharge device 4, FIG. 1, at aconstant speed in parallel with the surface of the insulative layer 2 inresponse to an input signal. During movement, the corona dischargedevice 4 operates a first microswitch, not shown, included in asynchronizing circuit 11 for energizing a current regulator 12 which isconnected to a time controller 13 including a plurality of variableresistors R R and R Variable resistor R is used to control the build-uprate of the corona discharge current between an interval of I or theinclination angle 0, until a current of a definite value I (see FIG. 5)is reached at the commencement of the discharge. Variable resistor R isused to maintain the current I at the constant value until an instant Iis reached, whereas the variable resistor R is used to control the rateof build-down of the corona discharge current between an interval t.,,that is the inclination angle 6 Thus, by the suitable adjustments ofvariable resistors R R and R it is possible to obtain any desiredcurrent waveforms as well as the discharge period.

The output from the current regulator 12 is applied to a source of highvoltage 14 so as to apply a variable voltage to the corona dischargedevice 4 to deposit a charge of the desired polarity (in this case,negative) on the surface of the insulative layer. The current flowingthrough the photosensitive element 30, or a body to be charged, is shownin FIG. 6, thus assuring uniform charge on the surface of the insulativelayer as will be described later. When the corona discharge terminatesat t.,, a signal is fed back from the output of current regulator 12 tosynchronizing circuit 11, thereby deenergizing current regulator 12,high voltage source 14 and corona discharge device 4. At this time thecorona discharge device 4 actuates a second micro-switch, not shown, fordeenergizing the driving circuit 10 for the corona discharge device 4.

In the control circuit shown in FIG. 4, the parameters for the coronadischarge device and of the body to be charged were selected to be thesame as the experiment described above and angles 6 and 6 were variedvariously by adjusting variable resistors R R and R In this experiment,the time scale on the abscissa was selected to be 0.1 sec/cm, thecurrent scale on the ordinate to be 20 mA/crn, the interval between tand to be 0.08 sec, and'the' current during this interval was varieduniformly from zero to 60 mA. The interval between t and was selected tobe 0.12 sec. and the current was maintained at a constant value duringthis interval. Further, the interval between and L, was selected to be0.3 sec. and during this interval the current was varied uniformly from60mA to zero. Under these conditions, it was noted that the surface ofthe insulative layer was charged uniformly when angle 0, was selected tobe more than 15 and angle 6 to be less than 45, and that the currentflowed through the insulative layer has varied as shown in FIG. 6. Ascan be noted from the graph shown in FIG. 6 there is no spike in thecurrent wave. I I

In another experiment, the speed of the corona discharge device was setto l 50 mm/sec. and angles 6 and 0 were varied under the same conditionsas the first mentioned experiment. With the same time and current scalesas described above, no nonuniform charge distribution on the surface ofthe insulative layer was, noted where angle 0 was larger than about andangle 0 was smaller than 55. As can be noted from these experiments, itis possible to increase angle 0 where the speed of movement of thecorona discharge device is increased.

In still another experiment, the control circuit was set to obtain anattenuation curve a shown in FIG. 7. With the time and current scalesdescribed above, it was confirmed that substantially uniformdistribution of the charge deposited on the surface of the insulativelayer can be obtained when the inclination angle 6 during currentattenuation was adjusted to lie between about 45 and 50. Further, nononuniform distribution of the deposited charge was noted when thecontrol circuit was set to obtain an attenuation curve b shown in FIG.7.

Further, with the time and current scales described above, an experimentwas performed in which the attenuation angle of the current wave flowingthrough the grounded substrate of corona discharge device was set to 45and the current was interrupted at an intermediate point of attenuation,as shown in FIG. 8. It was found that when the ratio of current B at thetime of interruption to the maximum current A was larger than l:3, astripe shaped nonuniform distribution of the deposited charge was notedand that substantially uniform distribution of the deposited charge wasobtained when the ratio was selected to be less than one-third.

Further, in accordance with this invention it is also possible to obtaina current waveform shown in FIG. 6 which is necessary to assure uniformcharge distribution by varying the distance between the corona dischargedevice and the surface of the body to be charged while maintaining at aconstant value as shown in FIG. 2, the current flowing through thegrounded substrate of the corona discharge device.

FIG. 9 diagrammatically shows the charging device utilized in thismodified method. As shown in this figure, opposite ends of a pair ofstationary parallel shafts and 16 are pivotally connected to a coronadischarge device 18 through parallel arms 17. The corona dischargedevice 18 may have the same construction as that of device 4 shown inFIG. 1. One end of a rope 19 is connected to one end of the coronadischarge device .18 and the opposite end of the rope 19 is connected toa drum 21 rotated by a motor 20. A spring 22 is interposed between thestationary portion of the copying machine and the opposite end of thecorona discharge device 18 for biasing the same to the left as viewed inFIG. 9.

After disposing a body to be charged or an insulative layer 23 beneaththe corona discharge device 18 and spaced apart therefrom a little andparallel therewith, motor 20 and drum 21 are rotated in thecounterclockwise direction. Then the corona discharge device will beswung about stationary shafts 15 and 16 to bring the corona dischargedevice 18 closer to the surface of the body to be charged 23 whilemaintaining them in parallel with each other. When current of a definitevalue is passed through the corona discharge device 18 during thismovement, it begins to discharge to commence deposition of charge of agiven polarity on the surface of the insulative layer. Upon furtherrotation of the drum 21, the arms 17 assume the vertical positionthereby decreasing to a minimum the distance between the coronadischarge device 18 and the insulative layer 23. Further rotation of thedrum 21 increases the distance. During this stroke, the operations ofthe corona discharge device and motor are terminated. Then, the spring22 pulls the corona discharge device 18 back to the original position.In this method of charging, the charging current flowing through theinsulative layer assumes a waveform as shown in FIG. 6, thus assuringsubstantially uniform charging.

With the arrangement shown in FIG. 9 an experiment was performed byusing a minimum distance of 7 mm between the corona discharge wires andthe surface of the insulative layer and a distance of 20 mm between themat the commencement and termination of the corona discharge, otherconditions being the same as in the first experiment. The result of thisexperiment showed that current having a waveform similar to that shownin FIG. 6 flowed through the insulative layer and that the chargedistribution was substantially uniform.

Although in the foregoing description, the corona discharge device wasmoved relative to the body to be charged, it will be clear that it isalso possible to move the body to be charged relative to the coronadischarge device as in the case of a rotary drum typeelectrophotographic copying machine. Further, although the body to becharged was shown as a three layered photosensitive element in one caseand as an insulative layer in the other case it will be clear that themethod and apparatus of this invention are applicable to any applicationrequiring uniform charging by corona discharge. The current controlcircuit shown in FIG. 4 may by replaced by a voltage control circuit forcontrolling the voltage of a high voltage source for the coronadischarge device. As has been pointed out hereinabove, the polarity ofthe charge is not limited to being negative.

We claim:

1. In a method of charging the surface of a body to be charged by meansof a corona discharge device including a plurality'of spaced coronadischarge wires and substantially covering the entire surface of saidbody by relatively moving said body and said corona discharge devicesubstantially parallel with each other, and wherein charging currenttends to increase at a high rate through said body at the commencementof the corona discharge and tends to decrease at a high rate at thetermination of the corona discharge, the improvement which comprises thesteps of controlling the rate of increase of said charging current to arate lower than said high rate of increase at the time of commencementof the corona discharge until a predetermined current value is reached,and controlling the rate of decrease of said current from saidpredetermined current value to a rate lower than said high rate ofdecrease at the time of termination of the corona discharge so as toprevent non-uniform charging of said surface caused by said coronadischarge wires.

2. The method according to claim 1 wherein said current is maintained atsaid predetermined value for a predetermined interval before it isdecreased.

3. The method according to claim 1 wherein the rate of current increaseis made to be higher at the time of commencement than the rate ofcurrent decrease at the time of termination of the corona discharge.

4. ln apparatus for charging the surface of a body to be charged bymeans of corona discharge of the type comprising a corona dischargedevice including a plurality of spaced corona discharge wires andsubstantially covering the surface of said body, a source of power forenergizing said corona discharge device, and means for moving saidcorona discharge device relative to said body in a directionsubstantially parallel to the surface of said body, and wherein chargingcurrent tends to increase at a high rate through said body at thecommencement of the corona discharge and tends to decrease at a highrate at the termination of the corona discharge, the improvement whichcomprises control means for controlling the rate of increase of saidcharging current to a rate lower than said high rate of increase at thetime of commencement of the corona discharge until a predeterminedcurrent value is reached and for controlling the rate of decrease ofsaid corona discharge current from said predetermined current value to arate lower than said high rate of decrease at the time of termination ofthe corona discharge so as to prevent nonuniform charging of saidsurface caused by said corona discharge wires.

5. The apparatus according to claim 4 wherein said control means furtherincludes means for maintaining said predetermined current value for apredetermined interval before said decrease of the corona dischargecurrent.

6. The apparatus according to claim 4 wherein said control meanscomprises an electric control circuit connected to said source of power,said electric control circuit including a plurality of variableresistors, a first one thereof determining the rate of increase of saidcorona discharge current, a second one determining the rate of decreaseof said corona discharge current and a third one determining saidpredetermined current value.

7. The apparatus according to claim 5 wherein said control meanscomprises mechanical means for driving said corona discharge devicetoward the surface of said body during the period of commencement of thecorona discharge and awayfrom said surface during the period oftermination of the corona discharge while said corona discharge deviceis being moved relative to said body in a direction substantiallyparallel to the surface of said body.

8. The apparatus according to claim 7 wherein said mechanical meanscomprises spaced parallel arms for suspending said corona dischargedevice.

1. In a method of charging the surface of a body to be charged by meansof a corona discharge device including a plurality of spaced coronadischarge wires and substantially covering the entire surface of saidbody by relatively moving said body and said corona discharge devicesubstantially parallel with each other, and wherein charging currenttends to increase at a high rate through said body at the commencementof the corona discharge and tends to decrease at a high rate at thetermination of the corona discharge, the improvement which comprises thesteps of controlling the rate of increase of said charging current to arate lower than said high rate of increase at the time of commencementof the corona discharge until a predetermined current value is reached,and controlling the rate of decrease of said current from saidpredetermined current value to a rate lower than said high rate ofdecrease at the time of termination of the corona discharge so as toprevent non-uniform charging of said surface caused by said coronadischarge wires.
 2. The method according to claim 1 wherein said currentis maintained at said predetermined value for a predetermined intervalbefore it is decreased.
 3. The method according to claim 1 wherein therate of current increase is made to be higher at the time ofcommencement than the rate of current decrease at the time oftermination of the corona discharge.
 4. In apparatus for charging thesurface of a body to be charged by means of corona discharge of the typecomprising a corona discharge device including a plurality of spacedcorona discharge wires and substantially covering the surface of saidbody, a source of power for energizing said corona discharge device, andmeans for moving said corona discharge device relative to said body in adirection substantially parallel to the surface of said body, andwherein charging current tends to increase at a high rate through saidbody at the commencement of the corona discharge and tends to decreaseat a high rate at the termination of the corona discharge, theimprovement which comprises control means for controlling the rate ofincrease of said charging current to a rate lower than said high rate ofincrease at the time of commencement of the corona discharge until apredetermined current value is reached and for controlling the rate ofdecrease of said corona discharge current from said predeterminedcurrent value to a rate lower than said high rate of decrease at thetime of termination of the corona discharge so as to prevent nonuniformcharging of said surface caused by said corona discharge wires.
 5. Theapparatus according to claim 4 wherein said control means furtherincludes means for maintaining said predetermined current value for apredetermined interval before said decrease of the corona dischargecurrent.
 6. The apparatus according to claim 4 wherein said controlmeans comprises an electric control circuit connected to said source ofpower, said electric control circuit including a plurality of variableresistors, a first one thereof determining the rate of increase of saidcorona discharge current, a second one determining the rate of decreaseof said corona discharge current and a third one determining saidpredetermined current value.
 7. The apparatus according to claim 5wherein said control means comprises mechanical means for driving saidcorona discharge device toward the surface of said body during theperiod of commencement of the corona discharge and away from saidsurface during the period of termination of the corona discharge whilesaid corona discharge device is being moved relative to said body in adirection substantially parallel to the surface of said body.
 8. Theapparatus according to claim 7 wherein said mechanical means comprisesspaced parallel arms for suspending said corona discharge device.